Of Dates and Time

First, a little background:

We live on a planet we call Earth. Long ago we as a society came to recognize the convention of breaking up our lives into a series of time units based on astronomical events, at the time having no other reliable means of marking it. The shortest of these we now call the day, and this time unit was once congruent with the amount of time it takes for our planet to execute one complete rotation. The second we call the month, which was once the same amount of time it took the moon to complete one revolution around our planet. And you know where I'm going with this, the longest one is the year, and the year used to be the amount of time it took for our planet to complete one revolution around the sun. For good measure, the ancient Israelites added another concept to the list: the week, which has nothing to do with astronomy.

In all of these cases except the week, I noted above that the actual amount of time related to them has changed. This came about as we became aware that to our dismay, the astronomical month, day, and year are not, and never have been, on speaking terms. The moon spins merrily around us, blissfully and inconsiderately unaware that the approximately twenty-seven days, seven hours, forty-three minutes, and twelve seconds that it takes to complete an orbit completely threw off our happy notion that we could divide the year up cleanly into twelve of these sections. Many attempts to correct this anomaly were devised. The Romans actually used to occasionally throw in an extra month after February called Mercedonius, which would would only take place every four years or so.

This became cumbersome. Fine, we said, we can live with this if we have to. No longer will the moon be relied upon for timing purposes. Unfortunately, by this time (Julius Caesar had about a year to live) the month as a handy concept to keep in your back pocket was too prevalent to dispose of entirely, so along came the Julian calendar, which among other things, defined that each month would have varying different number of days.

But what about those days? Again to our chagrin, we made the discovery that the earth's rotation and revolution have little to do with each other. Surely, we said, the earth and the sun can get together and come to some sort of integral ratio of days to years, can't they? But it wasn't to be. Caesar and Co. determined that there were about three hundred sixty-five and a quarter days to every year, and so instituted, along with their wacky month system, a leap year, during which good old February (or the "leftover month") would have an extra day every four years so that we stay synched up. To get the world lined up to start this Julian calendar, 46 BC had 445 days in it to make up the difference in centuries of measuring error. This year, quite appropriately, was called Annus Confusionus, or the "Year of Confusion."

Aside: The Romans also had a crazy naming convention for the days within a month. The Kalends was the name for the first day of the month, every month. The Nones was the fifth day or the eighth day of the month, depending on the month (March, May, July, and October had the eighth, and don't ask me why). Finally, the Ides marked either the thirteenth or the fifteenth days of the month in compliance with having the same number of days between the Nones and the Ides. So the Ides of March was March 15, while the Ides of April was April 13. They referred to the days in between in reference to the number of days prior to the next milestone. ante diem VI Nones Mart. was March 2, or "six days prior to the Nones of March."

Clearly, this system was too nasty to keep up with for long. Eventually, as the technology to measure time improved, we discovered that a mere extra day every four years was not accurate enough, as the year was actually about 365.2425 days long. So in 1582, Pope Gregory XIII instituted the Gregorian calendar, which made further corrections to the increasingly ridiculous leap year concept. Now leap years happen every four years except for years that are divisible by 100 and not divisible by 400. An interesting side effect of this ruling is that by papal decree, the day after October 4, 1582, was October 15, 1582. This was to curb the inaccuracies caused by 1600 years of the Julian calendar.

This is the system we have today, although we now know that the Gregorian calendar too is wrong. The year is not 365.2425 days long: it is 365.242375 days long. This means that we will accumulate an extra day every 8000 years or so. Also, given that the earth's rotation is slowing down gradually, we are increasingly becoming disconnected between the astronomical day and the length of time dictated by the International Bureau of Weights and Measures.

What should we do about this? Stay tuned.